1. Field of the Invention
The present invention generally relates to a magnetic random access memory (hereinafter, referred to as “MRAM”), and in particular, to an MRAM comprising a magnetic tunnel junction (hereinafter, referred to as “MTJ”) between a word line and a P-N diode, the MRAM wherein memory cells for storing two or more data are connected in series to each other in a form of an NAND, and data is read/written therein.
2. Description of the Prior Art
Most of companies for fabricating semiconductor memory are developing an MRAM using ferromagnetic materials, which is one of the next generation memory devices. An MRAM is a memory form for storing magnetic polarization in the thin film of magnetic materials. In the MRAM, read/write operations are performed by changing magnetic polarization according to the magnetic field generated by combining currents in a bit line and a word line.
The above MRAM is comprised of various kinds of cells such as a giant magneto resistance (hereinafter, referred to as “GMR”) or an MTJ. In other words, the MRAM is a memory device embodied by using GMR or spin polarization magnetic permeating phenomena. Those phenomena are generated due to the influence of spin on transmission of electrons. First, the MRAM using a GMR is embodied by using a phenomenon wherein resistance is more differentiated when spin directions are anti-parallel better than when parallel in two magnetic layers having an insulating layer therebetween. Second, the MRAM using spin polarization magnetic permeation is embodied by using a phenomenon wherein current is better permeated when spin directions are parallel than when anti-parallel in two magnetic layers having an layer therebetween.
The conventional MRAM has a structure of 1T+1MTJ comprising a switching device T and an MTJ, as shown in FIG. 1.
Here, FIGS. 2a and 2b represents the structure of the MTJ.
In detail, an MTJ includes a free ferromagnetic layer 2, a tunnel junction layer 3 and a fixed ferromagnetic layer 4. The free ferromagnetic layer 2 is formed on the top while the fixed ferromagnetic layer 4 is on the bottom. Here, a free ferromagnetic layer 2 and a fixed ferromagnetic layer 4 consists of NiFeCo/CoFe, and a tunnel junction layer 3 of Al2O3. The thickness of a free ferromagnetic layer 2 is different from that of a fixed ferromagnetic layer 4. According to this difference of thickness, magnetic polarization of a fixed ferromagnetic layer 4 is changed just in a strong magnetic field, while that of a free ferromagnetic layer 2 is changed even in a weak magnetic field.
FIG. 2a is a diagram illustrating an example of parallel magnetization orientation in a free ferromagnetic layer 2 and a fixed ferromagnetic layer 4. If the magnetization orientation is parallel, a sensing current increases. On the contrary FIG. 2b is a diagram illustrating an example of anti-parallel magnetization orientation in a free ferromagnetic layer 2 and a fixed ferromagnetic layer 4. In this case, a sensing current decreases. Here, magnetization orientation of a free ferromagnetic layer 2 is changed by an external magnetic field. An MRAM cell stores logic values of “0” or “1” according to this magnetization orientation of a free ferromagnetic layer 2. As a result, during a write operation, while magnetic polarization of a fixed ferromagnetic layer 4 is maintained, that of a free ferromagnetic layer 2 is changed.
As shown in FIG. 1, an MRAM cell includes a plurality of word lines WL1˜WL4, a plurality of bit lines BL1 and BL2, a cell 1 selected by those lines, and sense amplifiers SA1 and SA2 connected to a plurality of bit lines BL1 and BL2.
In the conventional MRAM cell having this structure, a cell 1 is selected by a word line WL4 selecting signal. When a predetermined voltage is applied to an MTJ through a switching device T, a sensing current flowing into a bit line BL2 is changed according to polarity of an MTJ. As a result, data can be read by amplifying this sensing current according to a sense amplifier SA2.
However, the above-described conventional MRAM has a complicated structure of a cell because a cell includes 1T+1MTJ. In other words, a process of embodying an MRAM is difficult because a cell has a transistor T and an MTJ. In addition, the conventional MRAM has a problem in a cell size.